ADSL technology is a popular choice for high-speed data transmission over existing telephone infrastructures. There are several reasons for this. One is that it operates over existing copper wires that are already present to many households and businesses. While some data communication infrastructures, such as fiber optic cable, may provide greater bandwidth, they require special lines to be run between the subscriber's location and a telephone company central office (“CO”). Some analog modems operating at 56 thousand bits per second (Kbps) do operate over conventional telephone infrastructure (i.e., copper wire from the home/business to the telephone company central office), but 56 Kbps is often unacceptably slow for users that require much higher bit rates for downloading video, graphics and other large datasets.
ADSL has the advantage that the data rates for ADSL can be 1.544 million bits per second (Mbps) or higher, using conventional copper wire. Another advantage of ADSL is that a telephone line is still available for use as a voice connection or for an analog modem connection when ADSL traffic is passing through the line. This is made possible by splitters at both ends of the line (at the subscriber end and at the CO) that separate the ADSL signals and conventional telephone signals at either end. Of course, unlike conventional analog modems, ADSL requires an ADSL modem at the CO end as well as at the subscriber end.
ADSL is a variation of DSL (Digital Subscriber Loop), the variation being that more of the available bandwidth is allocated to the down link (from the CO to the subscriber) than to the up link (form the subscriber to the CO) in recognition of the fact that the typical subscriber will download much more data than upload data. For example, in one configuration, an ADSL modem might provide a downstream link (CO to subscriber) of 6 Mbps and an uplink stream (subscriber to CO) of 640 Kbps.
With a conventional analog modem, the line is only in use when the analog modem dials in to the CO. Typically, the analog modem dials a number associated with a point-of-presence (POP) of the subscriber's service provider who, by a service agreement, agrees to answer the call, connect to the subscriber's modem and carry the subscriber's data traffic. A common arrangement today is the Internet service provider (ISP) with whom the subscriber subscribes, where the ISP carries the subscriber's data and connects the subscriber to the global internetwork of networks known as the “Internet”. Since the data traffic is carried through the telephone company infrastructure in the same manner as a telephone call, the telephone company does not need any special hardware to carry the data traffic, nor does the typical telephone company distinguish data traffic from voice traffic on the line. To the telephone company, the connection looks the same: the telephone customer goes “off-hook” (i.e., connects to their telephone line), dials a number, carries on a conversation with the answering party and, eventually, disconnects from the line.
By contrast, an ADSL connection modem is always “on” in that the line between two ADSL modems is in constant use. Since ADSL operations on a copper line do not interfere with its use as a voice line, the constant operation does not interfere with conventional use of the copper line. Another difference between ADSL and many other technologies for transmitting data over a telephone line is that ADSL requires ADSL hardware at the CO. In a typical arrangement, a subscriber's ADSL modem communicates with an ADSL modem at the CO and the CO ADSL modem connects to a data network (most commonly the Internet), thereby allowing the subscriber to connect to the data network over an existing telephone line. Since the CO will have one ADSL modem (or one segment of a multi-line ADSL card) for each subscriber, the telephone company must install considerable numbers of ADSL modems or cards to service those subscribers. Unlike the circuits the telephone company installs at the CO to carry telephone traffic from the CO to the telephone grid, where bandwidth is only allocated to telephone calls that are in progress, the ADSL modems at the CO are also “always on”. The telephone company might be able to operate with fewer ADSL modems than subscribers by taking advantage of the fact that some of the subscribers will power down their ADSL modems, but the telephone company still has significant investments in ADSL modems.
A host-signal processing (HSP) based ADSL modem implements most of its ADSL modulation functions via the computer processor. Therefore, it reduces the need for a separate high-speed digital signal processor (DSP). Through software implementation, it also has advantages for programming flexibility. Specifically, it allows an easy software update for improved protocols and algorithms for better performance and new standards requirement.
On the other hand, a HSP modem requires a large percentage of the CPU host power to perform the required ADSL and other functions. In general, the overall modulation functions of a typical full-rate ADSL DMT (discrete multi-tone) implementation defined by the ADSL standards (including ANSI T1.413 issue 2, ITU-T G.992.1) will require 300 MIPS. Thus, for a 300 MHz machine, ADSL driver will occupy 100% CPU usage, which is not acceptable.
To reduce this requirement, the technique so called “scalability” that allows the required CPU loaded to be adjusted according to the transmission rates has been introduced earlier (see SAM patents). As a result, we can reduce the required CPU load by operating the ADSL modem at a reduced rate.
Since ADSL is an “always-on” technology that performs physical layer modulation all the time, the ADSL transceiver normally performs all computation tasks whether or not actual data is being transferred. Although an ADSL transceiver can be disabled entirely during periods of inactivity, but since the training period at the start of an ADSL connection is quite long (usually about 20 seconds), users might find such delays intolerable.